Power amplifiers are used in a communication device, such as a radiotelephone, for amplifying radio frequency (RF) signals for transmission. One type of radiotelephone operates in the GSM (Group Special Mobile) radiotelephone system. In the GSM system, up to 8 radiotelephones simultaneously share a single channel on which to broadcast. Each of the radiotelephones is limited to transmitting RF signals during an allocated time slot of approximately 577 .mu.s. The radiotelephone must quickly ramp up its power amplifier to the proper frequency, transmit the RF signals, and quickly ramp down the power amplifier within the 577 .mu.s window. As a result, the output power of the power amplifier must be controlled with great accuracy to meet system requirements and not interfere with the other radiotelephones sharing the same channel. This control may be accomplished by employing a closed loop feedback structure, such as that disclosed in U.S. Pat. No. 5,150,075 by Hietala et al., entitled "Power Amplifier Ramp Up Method and Apparatus", issued on Sep. 22, 1992, and assigned to Motorola, Inc.
One widely used power amplifier comprises depletion mode transistors, such as gallium arsenide (GaAs) MESFET (metal-semiconductor field effect transistor) devices. GaAs MESFETs are beneficial because they exhibit comparatively high electron mobility and provide good performance across a wide frequency range. Because the GaAs MESFET is a depletion mode transistor, it requires application of a negative bias voltage to its gate for active mode biasing. The negative bias voltage is typically supplied by a level shifter coupled to the gate of the GaAs MESFET. Because the gate threshold voltage of a GaAs MESFET varies with changes in temperature, the level shifter must be adjustable to accordingly vary the negative bias voltage so as to maintain accurate control of the output power of the power amplifier. One such level shifter is disclosed in U.S. Pat. No. 5,559,471 by Black et al., entitled "An Amplifier and Biasing Circuit Therefore", issued on Sep. 24, 1996, and assigned to Motorola, Inc. In addition to being adjustable, it would further be desirable to minimize the temperature sensitivity of the level shifter itself to achieve additional accuracy in output power control.
Thus, what is needed is a level shifter for use in a power amplifier circuit for a communication device employing a closed loop feedback system that is not sensitive to temperature variations.